Binding of the H-ras p21 GTPase activating protein by the activated epidermal growth factor receptor leads to inhibition of the p21 GTPase activity in vitro.

There is strong, albeit indirect, evidence for a mitogenic signal transduction pathway comprising growth factors, growth factor receptors, the GTPase activating protein (p120-GAP), and p21ras. To demonstrate a direct physical association between these proteins in the absence of other cell constituents, their interaction was studied in vitro. Our results obtained with homogeneous protein preparations show that the activated epidermal growth factor (EGF) receptor phosphorylates p120-GAP at one site. Phosphorylated p120-GAP remains firmly bound to the receptor at physiological salt concentration; this leads to product inhibition of the receptor kinase activity as shown by diminished autophosphorylation activity and lack of turnover in p120-GAP phosphorylation. Phosphorylated p120-GAP is as active in stimulating the p21ras.GTPase as unphosphorylated GAP. p120-GAP, however, when bound to the EGF receptor is by a factor of 2 less active in stimulating the p21ras.GTPase than free p120-GAP. This effect might contribute to regulate the steady-state level of p21-GTP.